Though specialized to process particular kinds of stimulus energies and information, different sense modalities also share important principles of information processing. Understanding similarities and common principles, and the underlying mechanisms, forms this project's long-range goal. The present proposal brings techniques of human psychophysical measurement to bear on two topics: intensity summation and selective attention. The characteristics of summation, which is found in most if not all modalities, may not be fixed but depend on properties of stimulation: The quantitative rules governing, for example, interaural loudness summation can depend on the stimulus range. Using methods of intensity scaling, studies will determine whether stimulus range also modifies intensity summation in touch perception (e.g., summation of two frequency components of vibratory stimuli) and chemosensation (summation of gustatory and olfactory components in "flavors"); studies using intensity-comparison will seek to confirm these effects. Other studies will determine whether related changes take place in the discriminability of pairs of stimuli. The second main topic concerns the role of attention in selective processing of stimuli and stimulus dimensions. Focusing attention in hearing on a single frequency leads to depressed sensitivity to other, distant signal frequencies, thereby revealing the presence of individual channels ("attention bands"). Using detection and discrimination methods, studies will determine how similar selective processing characterizes touch, taste, and olfactory perception, evaluating attention bands for different qualities (e.g., different tastes), different spatial locations (e.g., tactile stimulation of different fingers), and different modalities (e.g., separate bands for olfactory and gustatory components in flavors). Related studies will determine how information from different channels combines. Attention can also guide processing of particular dimensions. Processing of perceptual information is often most efficient when stimuli vary on perceptually-primary dimensions. Studies will use discrimination methods to measure the ability to attend selectively to differences in distance, time, and velocity of moving visual stimuli. Lastly, although different modalities (e.g., vision and hearing) are often considered independent, tasks requiring rapid processing reveal intermodal interactions in response speed. Studies will determine whether such interactions are also evident in unspeeded tasks, using discrimination methods that provide separate measures of perceptual sensitivity and response criterion. In sum, this research program should shed important light on the basic operations of human perceptual systems. Not only can this improve our understanding of normal sensory-perceptual functioning in healthy individuals, but ultimately also disorders in individuals with sensory impairments and attentional deficits.